The Planets and Dwarf Planets

A planet is a large space body which reflects the light of a star around
which it
revolves. The planets in our solar system are classified as inner
planets and outer planets. The inner planets, the closest to the Sun, are
solid spheres of rock and include Mercury, Venus, Earth, and Mars. The inner
planets were constantly bombarded by
asteroids and
meteorites during their first 600 million years in existence.
Consequently, you will find craters of varying sizes on the inner planets and
their
satellites. The outer planets are large
gaseous spheres with rings and include Jupiter, Saturn, Uranus, and Neptune.
Between the inner and outer planets is an asteroid belt. Every planet, except
for Earth, was named for an ancient Roman god or goddess. Some of the planets
have naturally occurring satellites, or moons, while others do not. All eight planets
orbit the Sun in their own unique way.

Dwarf planet is a new class of astronomical objects. It was created in 2006 by the International Astronomical Union as part of their struggle to define the word 'planet'.
The discovery of objects in the outer solar system which were larger than or of similar size as Pluto necessitated the need for a definition. (This distant region is called
the Kuiper Belt and extends far beyond the orbit of Neptune.) After much debate, it was
decided that to be a planet in our solar system, an object must be in orbit around the Sun, have enough mass so that it has become round in shape due to its own gravity, and
have cleared out its orbital path around the Sun (so there are not similar objects to itself at roughly the same distance from the Sun). Dwarf planets were declared to be the
class of objects which met the first 2 requirements, but failed the third. It was then made clear that planets and dwarf planets are distinct classes of objects. Dwarf planets
are not planets.

Pluto, Ceres, and Eris became the first three members of this dwarf planet classification, and many others are expected to follow. In fact, there are about 70
other known objects which may be moved into this classification in the near future. More than a hundred other such objects are thought to still be lurking
undetected in the Kuiper Belt.